Gas generator for an air bag and air bag system

ABSTRACT

A gas generator for an air bag, in which conduction of heat generated by combustion of a gas generating agent is restricted to effectively reduce or inhibit a temperature increase of a diffuser side on an outer shell container, is provided and thus, reduces the maximum surface temperature of the diffuser. The gas generator includes a housing which has a diffuser shell with at least one gas discharge port and a closure shell for forming an accommodating space together with the diffuser shell and the gas generating agent, installed in the housing, ignited and burnt by an igniting unit to generate a combustion gas, wherein the gas generator further includes a heat transfer restricting member for making a conduction of heat generated by a combustion of the gas generating agent to the diffuser shell side smaller than that to the closure shell side.

[0001] This application is a divisional of co-pending Application No.09/339,225, filed on Jun. 24, 1999, the entire contents of which arehereby incorporated by reference and for which priority is claimed under35 U.S.C. § 120; and this application claims priority of Application No.10-179960, filed in Japan on Jun. 26, 1998 under 35 U.S.C. § 119.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a gas generator for an air bag,and to an air bag system.

[0004] 2. Description of the Related Art

[0005] An air bag system mounted on various vehicles such as automobileshas an object that, when the vehicle collides at a high speed, an airbag (bag body), which is rapidly inflated by gas, supports a vehicleoccupant so as to prevent the occupant from being injured in collidingwith hard portions of a vehicle interior such as a steering wheel and awindshield glass due to inertia of the occupant.

[0006] In general, such an air bag system comprises a gas generatorwhich is operated by a collision of a vehicle to discharge gas, and anair bag to which the discharged gas is introduced to be inflated.Conventionally, as such a gas generator, there has been provided a gasgenerator having a structure that gas generating means is, burnt togenerate combustion gas, is accommodated in the gas generator, and gasgenerated by the combustion of the gas generating means is dischargedfrom the gas generator.

[0007] In the gas generator for an air bag which generates combustiongas by a combustion of the gas generating means, an operating time ofthe gas generator is an extremely short, that is, about 60 milliseconds.However, since a combustion gas generated by the combustion of the gasgenerating means has high temperature, the following case is consideredafter the gas generator has been operated; more specifically, atemperature of an outer container (housing) of the gas generatorgradually increases due to conduction of heat generated by combustion ofthe gas generating means. In particular, in the case of a gas generatorincluding coolant means in its outer container so that high temperaturegas is not directly introduced into an air bag, the coolant means coolscombustion gas and combustion residues by heat exchange. For thisreason, the coolant means itself becomes hot, and then, heat of thecoolant means is transferred to the outer container; as a result, thetemperature of the outer container increases. The aforesaid gasgenerator is installed in a module case together with an air bag, and ismounted on various vehicles as an air bag system. In this case,considering safety after the gas generator has been operated, in the gasgenerator, it is desirable to prevent a temperature increase on a sidewhere the air bag is inflated, that is, on the diffuser shell side, asmuch as possible. This is because the following problem is considered.An air bag quickly deflates after inflating due to a collision. When anoccupant accidentally touches the diffuser shell, in the case where thesurface temperature on the diffuser shell side is too high, the occupantmay burn himself due to the increased temperature of the diffuser shellside.

[0008] However, there has not been provided a gas generator for an airbag, which effectively reduces or inhibits a temperature increase in anouter shell container on a diffuser side after the gas generator hasbeen operated (in particular, a temperature increase by conduction ofheat generated by combustion of the gas generating means).

[0009] Incidentally, in the past, International Publication No.W096/10495 discloses a gas generator having a structure in which a heatinsulating member is provided above and/or below an outer chamberincluding a gas generating agent. In the aforesaid gas generator, theheat insulating member is disposed between the gas generating agent andthe container (housing) in order not to affect a burning velocity of thegas generating agent by ambient temperature. In this case, the gasgenerator has an effect of preventing a temperature increase of ahousing, where the heat insulating member is provided, after the gasgenerator is operated. However, the surface temperature under the heatinsulating member is substantially high, and the heat insulating memberpeels from the outer container by gas jetting at a high speed. As aresult, there is the case where no achievement of the purpose is made.

SUMMARY OF THE INVENTION

[0010] In view of the above problem in the prior art, it is therefore anobject of the present invention to provide a gas generator for an airbag, which can restrict conduction of heat generated by combustion ofgas generating means after the generator is operated to effectivelyreduce or inhibit a temperature increase of a diffuser side in an outershell container, and thus, can reduce the maximum surface temperature ofthe diffuser.

[0011] According to the present invention, the diffuser is preventedfrom becoming too hot and the air bag can be used with safety.

[0012] To achieve the above object, the present invention provides a gasgenerator for an air bag comprising a housing which has a diffuser shellwith at least one gas discharge port and a closure shell for forming anaccommodating space together with the diffuser shell, and gas generatingmeans, installed in the housing, to be ignited and burnt by ignitingmeans and to generate a combustion gas, wherein the gas generatorfurther includes heat transfer restricting means for making a conductionof heat generated by combustion of the gas generating means to thediffuser shell side less than that to the closure shell side. In thecase that the coolant means for cooling the combustion gas generated bythe combustion of the gas generating means is accommodated in thehousing, it is desirable that the heat transfer restricting meansreduces or inhibits heat conduction from the coolant means to thediffuser shell. In general, the coolant means is used so as not todirectly introduce the combustion gas generated by the combustion of thegas generating means into the air bag. The conduction of heat generatedby combustion of the gas generating means to the diffuser shell afterthe gas generator has been operated is mainly made from the coolantmeans. For this reason, the heat transfer restricting means is disposedbetween the coolant means and the diffuser shell, and thereby, it ispossible to effectively reduce or inhibit heat conduction from thecoolant means.

[0013] The gas generator of the present invention includes the heattransfer restricting means, and thereby, it is possible to make themaximum surface temperature of the diffuser shell (i.e., the highesttemperature of a surface temperature range of the diffuser shell afterthe gas generating agent has been burnt) lower than that of the closureshell. The aforesaid heat transfer restricting means includes, forinstance, a heat transfer suppressing member disposed between thediffuse shell and a member such as the coolant means for conducting aheat generated by the combustion of the gas generating means to thediffuser shell, or a space formed between both components. The heattransfer suppressing member reduces or inhibits conduction of heatgenerated by combustion of the gas generating means from the coolantmeans and the like to the diffuser shell, and must function so that atleast heat generated by combustion of the gas generating means is notconducted to the diffuser shell. The aforesaid heat transfer suppressingmember includes, for instance, a heat insulating member formed by usinga proper heat insulating material, a space forming member for forming aheat insulating space or the like. Desirably, the heat insulating memberis sufficiently durable to a combustion temperature of the gasgenerating means and exhibits a heat insulating effect. Preferably, aceramic or ceramic fiber is used as the heat insulating member, inparticular. Moreover, a space formed by the space forming memberdisposed between the coolant means and the diffuser shell serves toreduce or inhibit a heat conduction from the coolant means to thediffuser shell. Therefore, preferably, the space is a heat insulatingspace at least capable of reducing or inhibiting a heat conductionbetween the coolant means and the diffuser shell. In particular, thespace forming member preferably has a structure where the heatinsulating space can be formed. In addition, in the case of using thespace formed between the coolant means and the diffuser shell as theheat transfer restricting means, the coolant means and the diffusershell do not directly make contact with each other, and no member ofconducting heat is disposed between both components. Thus, a heat of thecoolant means is not directly conducted to the diffuser shell, and isconducted thereto by only its radiation and/or convection. As a result,it is possible to restrict an increase of the surface temperature of thediffuser shell.

[0014] As described above, the heat transfer suppressing member isdisposed between the diffuser shell and a member such as the coolantmeans which absorbs heat generated by combustion of the gas generatingagent. In the case of interposing the heat transfer suppressing memberbetween the coolant means and an inner surface of a ceiling portion ofthe diffuser shell, in order to prevent a short-pass, in which acombustion gas passes between the heat transfer suppressing member andthe coolant means, and/or between the heat transfer suppressing memberand the diffuser shell, a short-pass preventing member, which covers anarea ranging from the upper inner surface of the coolant means to aninner surface of the ceiling portion of the diffuser shell, may beformed integrally with the heat transfer suppressing member. Forexample, in the case of using a heat insulating member as the heattransfer suppressing member, the heat insulating member and theshort-pass preventing member may be integrally formed. Also, in the caseof using a space forming member as the heat transfer restricting means,for example, the space forming member and the short-pass preventingmember may be integrally formed as shown in an Embodiment 1 of FIG. 1which will be described later.

[0015] In addition to the coolant means in the conventional case whichcomprises a metallic wire mesh for cooling combustion gas generated bycombustion of the gas generating means, the coolant means may alsoinclude the filter means as described above which is generally used forremoving residues in combustion gas and also performs a cooling functionof the combustion gas. Moreover, a coolant/filter, which has acomplicated spacing structure with the use of a multi-layer wire meshbody so as to cool combustion gas and collects combustion residues inthe combustion gas, may also be used. In general, each of these coolantmeans is formed into a substantially cylindrical shape, and is arrangedoutside a portion where the gas generating means is positioned. An outerperiphery of the coolant means may be provided with an outer layer whichcomprises a multi-layer wire mesh body, a multi-perforated cylinder, anannular belt or the like in order to prevent the coolant means frombeing bulged by the combustion of the gas generating means.

[0016] In the gas generator of the present invention, a conventionalazide based gas generating material(s) on the basis of inorganic azide,for example, natrium azide (sodium azide) can be used as the gasgenerating means which is contained in the housing and is burnt togenerate a combustion gas, and besides, a non-azide based gas generatingmaterial(s), which is not based on the inorganic azide can also be used.However, in general, the non-azide based gas generating material ishigher in combustion temperature than the azide based gas generatingmaterial, and the former has a higher calorific value permit gasgenerating quantity than the latter. Further, in the gas generator ofthe present invention, since the heat transfer restricting means canmake the maximum surface temperature of the diffuser shell lower thanthat of the closure shell, a further remarkable effect can be obtainedby using the non-azide based gas generating material as the gasgenerating means. Various materials have been proposed as the non-azidebased gas generating compositions. For example, the following materialsare known as the agent; more specifically, an organic compoundcontaining nitrogen, such as tetrazole, triazole, or metal salts ofthese, etc., and an oxidizing agent containing oxygen, such as alkalimetal nitrate, etc., and compositions using as their fuel and nitrogensource, triaminoguanidine nitrate, carbohydrazide, nitroguanizine, etc.,and using as their oxidizing agent the nitrate, chlorate, perchlorate,etc., of an alkali metal or alkali earth metal. These non-azide basedgas generating materials are applicable to the present invention;however, the gas generating agent of the present invention is properlyselected on demand of a burning velocity, non-toxicity and combustiontemperature, without being limited to these. The gas generating agent isused in a proper shape such as a pellet, a wafer, a hollow cylinder, adisk, a single or multiple perforated body, etc.

[0017] The housing is formed by casting, forging or pressing thediffuser shell having the gas discharge port and the closure shell forforming an accommodating space together with the diffuser shell, and byjoining both shells together. Joining of both shells is carried out byvarious welding methods, for example, electron beam welding, laserwelding, TIG arc welding, projection welding or the like. In the casewhere the diffuser shell and the closure shell are formed by pressingvarious steel plate and sheet such as stainless copper plate, it ispossible to readily manufacture both shells, and to achieve a reductionin its manufacture cost. Moreover, these shells are formed into acylinder, that is, a simple shape, and thereby, its pressing is easy.Preferably, a stainless steel plate is used as the material of thediffuser shell and the closure shell, and a copper plate with nickelplating may be used. Incidentally, an interior space of the housing isdivided into two chambers by providing the inner cylindrical memberinside the housing, and thereafter, various members may be properlycontained therein.

[0018] Further, the housing is usually provided therein with ignitingmeans which is operated when an impact is detected, and ignites andburns the gas generating means. The igniting means may be either of amechanical ignition type igniting means which is operated upon sensingan impact exclusively by means of a mechanical device or mechanism, andan electric ignition type igniting means which is operated by anelectrical signal transmitted from an impact sensor for sensing animpact. Preferably, the electric ignition type igniting means is used asthe igniting means. The electric ignition type igniting means comprises:an electrical sensor which senses an impact exclusively by an electricalmechanism; an igniter which is operated by an electrical signaltransmitted from the sensor upon sensing an impact; and a transfercharge ignited by the operation of the igniter and then burns. As theelectrical sensor, there is, for example, a semiconductor typeacceleration sensor or the like.

[0019] The aforesaid gas generator for an air bag is accommodated in amodule case together with an air bag (bag body) to which gas generatedby the gas generator is introduced to expand the bag, and thus, an airbag system is constructed. In the air bag system, the gas generator isoperated at the point of time when the impact sensor senses an impact,and combustion gas is discharged from the gas discharge port of thehousing. Then, the combustion gas flows into the air bag, and thereby,the air bag breaks the module cover and inflates, and thus, forms acushion for absorbing an impact between hard structures in a vehicle anda vehicle occupant.

[0020] Further, to achieve the above object, the present inventionprovides a method for reducing temperature increase of a diffuser shellin a gas generator for an air bag comprising a housing which has adiffuser shell with at least one gas discharge port and a closure shellfor forming an accommodating space together with the diffuser shell andgas generating means, installed in the housing, to be ignited and burntby igniting means and to generate a combustion gas, wherein a conductionof heat generated by combustion of the gas generating means to thediffuser shell is reduced or inhibited to reduce a temperature increaseof an outer surface of the diffuser shell by combustion of the gasgenerating means.

[0021] The conduction of combustion gas of the gas generating means tothe diffuser shell is reduced or inhibited by interposing a spaceforming member and a heat transfer suppressing member such as a heatinsulating member between the diffuser shell and a coolant means. Thecoolant means may be a coolant/filter which is formed to have acomplicated spacing structure with the use of the multi-layer wire meshbody.

[0022] The present invention provides a gas generator for an air bag,which can effectively reduce or inhibit a temperature increase on adiffuser side by heat generated by combustion of gas generating meansafter the gas generator has been operated, and thus, can reduce themaximum surface temperature of the diffuser.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a longitudinal sectional view showing a preferredembodiment of a gas generator for an air bag of the present invention;

[0024]FIG. 2 is a cross-sectional view of main parts showing anotherembodiment of a space forming member in shown in FIG. 1;

[0025]FIG. 3 is a cross-sectional view of main parts showing anotherembodiment of a heat transfer restricting means;

[0026]FIG. 4 is a cross-sectional view of main parts showing stillanother embodiment of the heat transfer restricting means;

[0027]FIG. 5 is a cross-sectional view of main parts showing stillanother embodiment of the heat transfer restricting means; and

[0028]FIG. 6 is a view showing a configuration of an air bag system ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] A gas generator for an air bag of the present invention will bedescribed below with reference to preferred embodiments shown in theaccompanying drawings.

[0030] [Embodiment 1]

[0031]FIG. 1 is a longitudinal sectional view showing a first embodimentof a gas generator for an air bag of the present invention.

[0032] As shown in FIG. 1, a gas generator includes a housing 3comprising a diffuser shell 1 and a closure shell 2. An interior of thehousing 3 is divided by an inner cylindrical member 13 into twochambers, that is, an igniting means accommodating chamber 23 and a gasgenerating agent combustion chamber 22. The igniting means accommodatingchamber 23 is provided, therein, with an igniter 4 and a transfer charge5, as igniting means, which ignites and burns the gas generating agent 6upon its activation when an impact is detected. On the other hand, thecombustion chamber 22 is provided therein with gas generating agent 6which is ignited and burnt by the igniting means to generate acombustion gas, and a substantially disk-like under plate 18 whichsupports the gas generating agents 6 and prohibits a movement of theseagents. A circumferential wall portion 10 of the diffuser shell 1 isformed with a plurality of gas discharge ports 11 in equal intervals ina circumferential direction thereof, and each gas discharge port 11 isclosed by a seal tape 25. In the closure shell 2, an inner cylindricalmember 13 is arranged such that it is fitted into a central opening 12of the closure shell 2. The diffuser shell 1 and the closure shell 2 arejointed together by laser welding in a state of mutually overlappingtheir flange portions 15 and 16 in the vicinity of the middle positionin cross section in an axial direction of the housing 3, and thus, formthe housing 3.

[0033] Moreover, in order to purify and cool gas generated by ignitionand combustion of the gas generating agent 6, a coolant/filter 7 isdisposed in the housing 3. The coolant/filter 7 surrounds the gasgenerating agents 6 such that an annular chamber, that is, the gasgenerating agent combustion chamber 22 is defined between the innercylindrical member 13 and the coolant/filter 7. Further, thecoolant/filter 7 is formed by overlapping a plain stitched wire meshmade of stainless steel in a radial direction, and compressing the wiremesh in radial and axial directions. As shown in FIG. 1, a space formingmember 50 is arranged at an end face 28 of the coolant/filter 7 on thediffuser shell 1 side, and thus, the coolant/filter 7 is formed shorterin the axial direction thereof than the height of the housing 3. On anouter side of the coolant/filter 7, an outer layer 24 is formed tofunction as suppressing means for suppressing an expansion of thecoolant/filter 7. In the case of forming the coolant/filter 7 shorter inthe axial direction, it is desirable that the outer layer 24 is alsoformed shorter in the axial direction likewise the coolant/filter 7. Agap 9 is formed between an outer peripheral wall 8 of the housing 3 andthe coolant/filter 7 to function as a gas passage.

[0034] The space forming member 50 is disposed between thecoolant/filter 7 and an upper inside 29 of the diffuser shell 1 as aheat transfer restricting means for making conduction of heat generatedby a combustion of the gas generating agent to the diffuser shell sideless than that to the closure shell side. The space forming member 50comprises a space forming portion 51 which is formed in a manner that anouter periphery of a ring-like member is curved in a plain direction,and a short-pass preventing portion 52 which covers an inner peripheralsurface of the upper portion (i.e., diffuser shell side) of thecoolant/filter 7. The space forming portion 51 is held between thecoolant/filter 7 and the diffuser shell 1, and thereby, a space 53 isformed between these members. The space 53 functions as a heatinsulating space for inhibiting a heat conduction from thecoolant/filter 7 to the diffuser shell 1. Moreover, the short-passpreventing portion 52 continuously covers an area ranging from the innerperipheral surface of the coolant/filter 7 to an upper inside 29 of thediffuser shell 1, and thereby, it is possible to prevent a short-pass,that is, prevents a combustion gas from passing through a portionabutting the coolant/filter 7 against the space forming portion 51 andthrough a portion abutting the upper inside 29 of the diffuser shell 1against the space forming portion 52. FIG. 2 is an enlargedcross-sectional view showing main parts of the space forming member 50.As shown in FIG. 2, the space forming portion 51 of the space formingmember 50 is held between the upper inside 29 of the diffuser shell 1and the end face 28 on the diffuser shell side of the coolant/filter 7;on the other hand, the short-pass preventing portion 52 is fitted intoan inner periphery of the coolant/filter 7. Also, in place of the spaceforming member 50, a space forming member 54, as shown in FIG. 3, may beused. The space forming member 54 is formed into a substantiallyrecessed shape in its longitudinal cross section, and by a recessedportion 55 of the space forming member 54, a space 56 is formed betweenthe coolant/filter 7 and the diffuser shell 1. The space 56 shown inFIG. 3 also reduces or inhibits heat conduction from the coolant/filter7 to the diffuser shell 1; therefore, after the gas generating agent isburnt, the maximum surface temperature of the diffuser shell 1 becomeslower than the maximum surface temperature of the closure shell 2.

[0035] An inner periphery of the coolant/filter 7 may be provided with asubstantially cylinder-like perforated basket (not shown) which protectsthe coolant/filter 7 from a flame generated by the combustion of the gasgenerating agent 6, and prevents the gas generating agent and thecoolant/filter 7 from directly contacting with each other.

[0036] In the housing 3, the substantially disk-like under plate 18 isarranged in the combustion chamber 22 defined outside the innercylindrical member 13. Further, the under plate 18 has a circularportion 19 which comes in contact with the gas generating agent 6, and acentral opening 20 for fitting an outer peripheral wall of the innercylindrical member 13 thereto. The circular portion 19 supports the gasgenerating agents 6 to inhibit movement of the gas generating agents 6,and thereby, there is no possibility that the gas generating agent 6breaks due to vibration, varying surface area of the gas generatingagent 6. A crimping portion 21 is formed at an end portion on a sideincluding the igniter 4 of the inner cylindrical member 13 to fix theigniter 4. Moreover, a peripheral wall of the inner cylindrical member13 is formed with a plurality of flame transferring ports 26 at equalintervals. Each flame transferring port 26 is closed by a seal tape 27.In this embodiment, the aforesaid outer layer 24, the gap 9 and theperforated basket is provided as the need arises, and may be omitted.

[0037] In the gas generator shown in FIG. 1, when the transfer charge 5is ignited and burnt by the igniter 4 operated upon impact, the flamebreaks the seal tape 27 that closes the flame transferring ports 26 ofthe inner cylindrical member 13, and then, flows into the combustionchamber 22. Whereupon, the gas generating agent 6 in the combustionchamber 22 is ignited and burnt by the flame of the transfer charge 5,and then, generates a combustion gas. The combustion gas is purified andcooled while passing through the coolant/filter 7, and then, passesthrough the gap 9. Further, the combustion gas breaks the seal tape 25closing the gas discharge port 11, and then, is discharged from the gasdischarge port 11. On the other hand, the coolant/filter 7 cools thecombustion gas and combustion residues by heat exchange; for thisreason, the temperature of the coolant/filter increases. However, theheat of the coolant/filter 7 is not conducted to the diffuser shell 1because a space 53 is formed by the space forming member 50 between thediffuser shell 1 and the coolant/filter 7. Therefore, it is possible tomake the maximum surface temperature of the diffuser shell 1, after thegas generating agent 6 is burnt, lower than that of the closure shell 2.

[0038] The heat transfer restricting means explained based on that thespace forming member makes a heat conduction on the diffuser shell sidegenerated by the combustion of gas generating agent less than that onthe closure shell side. In addition to the space forming members asshown in FIG. 1 to FIG. 3, the aforesaid heat transfer restricting meansincludes another embodiments as shown in FIG. 4 and FIG. 5. The heattransfer restricting means according to these embodiments will bedescribed below with reference to the drawings.

[0039] Each heat transfer restricting means as shown in FIG. 4a and FIG.4b is a heat insulating member for inhibiting heat conduction from thecoolant/filter 7 to the diffuser shell 1, and is disposed between thecoolant/filter 7 and the diffuser shell 1. Thus, the heat of thecoolant/filter 7, which is increased when cooling a combustion gas ofthe gas generating agent is not conducted to the diffuser shell 1. As aresult, the temperature increase of the diffuser shell 1 is prevented,and therefore, it is possible to make the maximum surface temperature ofthe diffuser shell lower than that of the closure shell. A heatinsulating member 57 shown in FIG. 4a is formed into a ring shape havinga size corresponding to the end face 28 of the coolant/filter 7. On theother hand, a heat insulating member 58 shown in FIG. 4b is formed likea ring shape having a size larger than the end face 28 of thecoolant/filter 7, and has a shape corresponding to a concave and convexshape of the upper inner side 29 of the diffuser shell 1. These heatinsulating members 57 and 58 comprise a ceramic or ceramic fiber; morespecifically, a material which is sufficiently durable to a combustiontemperature of the gas generating agent and exhibits a heat insulatingeffect.

[0040] A heat transfer restricting means of another embodiment shown inFIG. 5 is a space 60 formed between the coolant/filter 7 and thediffuser shell 1. The space 60 is formed such that a heat of thecoolant/filter 7 is not directly conducted to the diffuser shell 1. As aresult, in the gas generator having the aforesaid space 60, the heat ofthe coolant/filter 7 is not conducted to the diffuser shell 1 except byradiation. Therefore, the heat of the coolant/filter 7 is preferentiallyconducted to the closure shell 2.

[0041] Moreover, the gas generator shown in FIG. 5 is different from thegas generator shown in FIG. 1 in that an inner cylindrical member 131does not directly make contact with the diffuser shell 1. Thus, theshort-pass preventing means 61, which supports the coolant/filter 7,comes in contact with the inner cylindrical member 131; for this reason,the heat of the coolant/filter 7 is conducted to the short-passpreventing means 61 and the inner cylindrical member 13′, but is notdirectly conducted to the diffuser shell 1. Also, a diffuser shell sideof the inner cylindrical member 13′ is closed such that a flame from thetransfer charge 5 is not directly discharged to the diffuser shell 1side. The short-pass preventing means 61 has a disk shape covering aportion where the gas generating agent 6 is accommodated and the endface of the coolant/filter 7, and further, is formed into a curved shapeto abut against an upper inner peripheral surface of the coolant/filter7.

[0042] Like reference numerals are used to designate other members sameas FIG. 1, and an explanation of these members is omitted.

[0043] [Embodiment 2]

[0044]FIG. 6 is a view showing a configuration of an air bag system ofthe present invention including a gas generator using an electricignition type igniting means.

[0045] The air bag system comprises a gas generator 200, an impactsensor 201, a control unit 202, a module case 203, and an air bag 204.The gas generator described based on FIG. 1 is used as the gas generator200. An operating performance of the gas generator 200 is adjusted sothat an impact given to a vehicle occupant is as little as possible inthe initial stage when the gas generator is operated.

[0046] The impact sensor 201 comprises a semiconductor type accelerationsensor, for example. The semiconductor type acceleration sensor has foursemiconductor strain gages formed on a silicon substrate beam which isdeflected when acceleration occurs. These semiconductor strain gages arebridge-connected. When acceleration occurs, the beam is deflected, andthen, the surface becomes strained. A resistance of the semiconductorstrain gage changes by the strain, and this change in resistance isdetected as a voltage signal which is proportional to the acceleration.

[0047] The control unit 202 includes an ignition discriminator circuit,and a signal from the semiconductor type acceleration sensor is inputtedto the ignition discriminator circuit. At the point of time when animpact signal from the impact sensor 201 exceeds a predetermined value,the control unit 202 starts a computation. When a computation resultexceeds a predetermined value, the control unit 202 outputs an operatingsignal to the igniter 4.

[0048] The module case 203 is made of polyurethane, for example, andincludes a module cover 205. The air bag 204 and the gas generator 200are installed inside the module case 203 so as to constitute a padmodule. In general, the pad module is attached to a steering wheel 207in the case of being mounting to a driver's seat side of an automobile.

[0049] The air bag 204 is made of nylon (e.g., nylon 66) or polyester,and is fixed to a flange portion of the gas generator in a state that anair bag opening 206 encloses the gas discharge ports of the gasgenerator while being folded.

[0050] When the semiconductor type acceleration sensor 201 senses animpact at the time of an automobile collision, an impact signal istransmitted to the control unit 202, and then, the control unit 202starts a computation at the point of time when the impact signal fromthe sensor exceeds a certain value. Then, the control unit 202 outputsan operating signal to the igniter 4 of the gas generator 200 when thecomputation result exceeds a certain value. In this manner, the igniter4 is operated to ignite the gas generating agent, and the gas generatingagent burns and generates gas. The gas is ejected into the air bag 204,and thereby, the air bag 204 breaks the module cover 205 and inflates toform a cushion for absorbing an impact between the steering wheel 207and the occupant.

EXPERIMENTAL EXAMPLES

[0051] With the use of the gas generator including the heat transferrestricting means under conditions shown in the following experimentalexamples, the maximum surface temperature of diffuser shell when the gasgenerating agents are burnt was measured, and then, was compared withthe maximum surface temperature of the diffuser shell when the gasgenerating agents of the gas generator having no heat transferrestricting means are burnt. The comparative results are shown in thefollowing Table 1.

[0052] This maximum surface temperature measurement was made in a mannerof pressing a K thermocouple (i.e., alumel-chromel thermocouple) to thesurface of the gas generator container. In the case of the gas generatorincluding no heat transfer restricting means, a portion of the diffusershell contacting with the coolant means had the maximum temperature.

Experimental Example 1

[0053] Condition: As shown in FIG. 3, a heat insulating member made ofstainless steel was disposed as the heat transfer restricting meansbetween the end portion of the coolant/filter and the inside of thediffuser shell, a temperature of the diffuser shell when the gasgenerating agents are burnt was measured.

Experimental Example 2

[0054] Condition: As shown in FIG. 1, the space forming member wasdisposed as the heat transfer restricting means between the end portionof the coolant/filter and the inside of the diffuser shell to form aheat. insulating space therebetween, a temperature of the diffuser shellwhen the gas generating agents are burnt was measured. TABLE 1 None ofheat Including heat transfer transfer Temperature restrictingrestricting difference means(° C.) means(° C.) (° C.) Experimental 355345 10 Example 1 Experimental 330 25 Example 2

[0055] [Experimental Results]

[0056] According to the present embodiment, in the case of the gasgenerator including the heat transfer restricting means, the maximumsurface temperature of the diffuser shell became lower as compared tothe case of the gas generator including the heat transfer restrictingmeans.

What is claimed:
 1. A gas generator for an air bag, comprising: ahousing having a top plate, a bottom plate, and a circumferential outerwall connecting the top plate and the bottom plate; a tubular memberprovided within said housing, said tubular member storing ignition meanstherein and being fixed to the bottom plate; coolant means, providedwithin said housing and outside the tubular member, for coolingcombustion gas generated by gas generating means provided inside saidhousing; and a contact preventing member, provided in a first spacedefined by an upper peripheral portion of said coolant means and aninner surface of said top plate opposing the upper peripheral portion,for preventing the upper peripheral portion of said coolant means frommaking direct contact with the top plate to restrict heat conductionfrom said coolant means to the top plate after activation of the gasgenerator, said contact preventing member having a circular portionprovided with a bent portion that engages with an outer peripheralsurface of the tubular member.
 2. The gas generator of claim 1, whereinsaid contact preventing member prevents combustion gas from by-passingbetween the upper peripheral portion of said coolant means and a lowerportion of said contact preventing member.
 3. The gas generatoraccording to claim 1, wherein a second space is defined by an outerperipheral portion of said coolant means and an inner surface of saidcircumferential outer wall.
 4. A method of restricting an increase intemperature of a housing of a gas generator after activation thereof,the housing including a top plate, a bottom plate, and a circumferentialouter wall connecting the top plate and the bottom plate, comprising:providing a tubular member within the housing for storing an ignitionunit therein; providing a coolant unit within the housing and outsidethe tubular member for cooling combustion gas generated by gasgenerating unit provided inside the housing; providing a first spacedefined by an upper peripheral portion of the coolant unit and an innersurface of the top plate opposing the upper peripheral portion;providing a contact preventing member inside the first space to preventthe upper peripheral portion of the coolant unit from making directcontact with the top plate such that heat conduction from the coolantunit to the top plate after activation of the gas generator isrestricted; providing the contact preventing member with a circularportion having a bent portion; and engaging the bent portion with anouter peripheral surface of the tubular member.
 5. The method of claim4, further comprising: preventing the combustion gas from by-passingbetween the upper peripheral portion of the coolant unit and a bottomsurface of the contact preventing member.
 6. The method of claim 4,further comprising; providing a second space defined by an outerperipheral portion of the coolant unit and an inner surface of thecircumferential outer wall.
 7. A gas generator for an air bag,comprising: a housing having a top plate, a bottom plate, and acircumferential outer wall connecting the top plate and the bottomplate; and coolant means provided within said housing, wherein said topplate and said coolant means define an empty space between an innersurface of the top plate and an upper peripheral surface of said coolantmeans to restrict heat conduction from said coolant means to said topplate.
 8. A gas generator for an air bag, comprising: a housing having atop plate, a bottom plate, and a circumferential outer wall connectingthe top plate and the bottom plate; coolant means provided within saidhousing, a contact preventing member provided inside an empty spacedefined by an inner surface of the top plate and an upper peripheralsurface of said coolant means, an upper surface of said contactpreventing member being spaced apart from the inner surface of the topplate.